This invention relates to metallized film for electrical capacitors.
Referring to FIG. 1, film capacitors are typically constructed by overlaying a pair of metallized films 1, 2 with a slight sideways displacement before winding the films to form a capacitor core. Opposite ends of the core are sprayed with a metal to form electrical terminals 4. The electrical terminals are soldered to wires forming the capacitor's external electrical contacts. The capacitor core is then immersed in a dielectric liquid within a housing.
Each metallized film comprises a very thin layer of metal 3 (e.g., zinc) deposited on a dielectric film 3b (e.g., polypropylene). The metal layer typically covers the entire surface of the dielectric film except along one longitudinal edge (the "unmetallized margin" 5). The unmetallized margin maintains electrical isolation between the metal layers on metallized films 1, 2, and ensures that each terminal 4 comes into contact with only one metallized film.
Standard setting organizations have long required that film capacitors include a resistor connected in parallel (the "UL resistor"). This is conventionally provided, at significant manufacturing cost, by attaching a separate resistor on the outside of the capacitor housing.
The ability of a film capacitor to store electrical energy is limited by the electric field strength that can safely be applied across the dielectric film. In recent years, there has been a demand to operate metallized film capacitors at higher electric fields (or voltages) and to miniaturize the capacitors by using thinner films (which also increases the strength of internal electrical fields in the capacitor). However, as the field strength rises, the frequency of dielectric faults in the capacitor increases.
A dielectric fault occurs when the dielectric strength at a particular location is insufficient, and arcing occurs through the dielectric film from one metal layer to the next. A dielectric fault may be the result of, for example, foreign particles entering the capacitor winding or local imperfections in the dielectric film, itself.
Infrequent, isolated faults can be accommodated by a process known as self-clearing or self-healing. The metallizing in the immediate vicinity of the fault is vaporized by the heat generated by the large currents flowing to the site of the fault. Conventional film capacitors operate with an electric field strength (or electrical stress) of between 60 and 84 V/micron. Operating above a capacitor's design field strength will produce faults so frequently that they cannot successfully be cleared, resulting in the capacitor overheating and failing.
During self-clearing, two competing processes are at work. Heat generated by current flowing toward the fault is causing metal to vaporize around the fault, and thus isolate it. But at the same time the heat and arcing in the vicinity of the fault is carbonizing the surrounding dielectric (e.g., creating conductive, carbon tracks), making it conductive, and thereby increasing the flow of current. A capacitor fails when the dielectric faults occur so frequently and so close to one another, and the amount of energy released in the clearing process is so high, that vaporization of metal around the faults does not occur quickly enough to halt the heating and carbonization of the dielectric.
In addition to causing dielectric faults, strong electrical fields may cause arcing, or corona discharges, between a terminal 4 and a metal layer 3 across the unmetallized margin 5 in the capacitor core. The corona discharge causes a deterioration of an edge of the metal layer adjacent the margin, most likely by oxidizing the metal layer. The deterioration progresses gradually inward from the edge, causing a portion of the metal layer to become insulating. The resulting decrease in the area of the metal layer causes a reduction in the capacitance of the capacitor.
The strength and frequency of arcing in the capacitor across the unmetallized margin typically overshadows the effects of dielectric faults in the capacitor, since the breakdown voltage between metal layers in the capacitor is generally much higher than voltages applied to the capacitor. Deterioration due to corona discharge across the margin is thus a limiting factor in operating the capacitor at either high voltages or with thinner films.
Several schemes attempt to prevent arcing across the unmetallized margin by reducing the electric field at the edge of the metal layer adjacent the margin. For example, in Japanese Patent No. 50-8050, the metal layer 3 is made thicker near the unmetallized margin (at 3') (FIG. 2). In another scheme, shown in FIG. 3 and described in Japanese Patent No. 50-85860, a semiconductor layer 7 made of Si or Ge is formed over the edge of the metal layer 3 adjacent the margin 5. In Japanese Patent No. 50-85861 (FIG. 4), an oxidized layer 8 is added between the edge of the metal layer 3 and a semiconductor layer 7. Japanese Patent No. 51-84061 (FIG. 5) describes adding a semiconductor layer 7 at the edge of the metal layer 3 below a high resistance semiconductor layer 9.
In a variation of the above schemes, Japanese Patent No. 50-45264 (FIG. 6) describes a capacitor having a metallized film 10 with a metal layer 3, 12 on each of its surfaces. The metallized film 10 has a central unmetallized margin 5. The second film 11 in the capacitor is non-metallized. An edge of film 11 and metal layer 12 are covered with a semiconductor layer 7, while the central margin 5 remains uncovered.
In all the schemes described above, the semicondutor layer does not extend into the unmetallized margin. As a result, the isolation provided by the unmetallized margin strongly limits the degree to which the electric field at the edge of the margin is reduced by the semiconductor layer.